Electrophotography refers to producing photographic images by electrical means, and xerography is a form of electrophotography for copying documents and other graphic matter. Xerographic copiers are extensively used in a variety of environments, such as offices, libraries, and educational institutions.
The basic elements of a xerographic copier are well known to those skilled in the art. A light source forms an electrostatic latent image of an original document on a photosensitive medium. The photosensitive medium, as it moves within the copier, travels adjacent to a source of tiny plastic particles called toner. The electrostatic force of the latent image on the photosensitive medium attracts the toner, thereby providing a developed image of toner particles on the surface of the photosensitive medium. The toner image is transferred through electrostatic charges to an image receptor, which is normally a sheet of paper or plastic. The image receptor then passes through a fuser which heats and melts the toner particles, thereby fixing or fusing the image of the original onto the image receptor.
Prior art fusers generally comprise a pair of rollers between which the image receptor passes. One roller, usually the bottom roller, is not as compressible as the other such that a nip is formed in the center of the contact length of the two rollers. The image receptor, while passing through the nip, traverses the arc of the less compliant roller. One or both of the rollers is heated so as to melt the toner particles. The two rollers compress the image receptor as it passes between the rollers, thereby fixing or fusing the melted toner particles to the image receptor. Oil is applied to the roller which makes direct contact with the plastic toner particles, so as to prevent the melted toner particles from adhering to the roller. In some prior art fusers, the fuser oil is applied to a third roller called an oiling roller which in turn applies the fuser oil to the fixing roller. The oiling roller is used to provide a more uniform thickness of fuser oil on the fixing roller.
The fuser oil is normally applied to the fuser with a wick. A portion of the wick is immersed in an oil reservoir from which the oil moves by capillary attraction through the wick to the oiling roller. In other prior art fusers, a wick applies the fuser oil directly to the fuser. A leveling blade contacting the oiled roller is often used to provide a more uniform thickness of fuser oil on the fixing roller.
In color xerographic copiers, to form a complete color image the photosensitive medium must form a separate image for each color of toner used (usually primary colors) and transfer these separate images, one at a time, to a second medium, where the different colors are superimposed one upon the other. This second medium is called a transfer medium. After the complete color image is formed on the transfer medium, the complete color image is transferred through electrostatic changes to the image receptor. The image receptor is then passed through a fuser.
Varying combinations of the individual color toner components are often necessary to obtain a multi-color image with accurate color tones. The superimposed toner images often produce stacks of toner having a varying number of toner layers on the surface of an image receptor. Consequently, the stacks of toner have varying thicknesses. The image receptor, usually paper or plastic, also does not have a regular surface, either. Paper, particularly, has much thickness variation due to its fibrous content. The stacks of toner and the composition of the image receptor produce a toner-laden image receptor having an uneven surface shaped with peaks and valleys. All of the layers of toner must be melted and fused to obtain accurate color tones. Because the surface of the toner-laden image receptor is irregular, prior art fusers tend to contact only the peaks of toner on the image receptor. Accordingly, the peaks of toner are melted and fused to the image receptor while the valleys of toner are not melted and fused. The fused toner then has a glossy finish while the unfused toner has a dull finish. As a result, the final multi-color toner image has a non-uniform finish. In addition, the thinner stacks of toner layers are not melted and mixed and thus the final colors of those thinner stacks are not accurate.
Non-fused toner is a particularly critical problem when the image receptor is a transparency. If the toner image on the transparency is not completely fused, light cannot pass through the areas of non-fused toner. The areas of non-fused toner are thus projected as black images.
The problems caused by the irregular surface of the toner-laden image receptor sheet are magnified during two-side or duplex printing where toner images are produced on both sides of the image receptor sheet. The number of peaks and valleys are roughly doubled because the stacks of toner layers are on both sides of the image receptor sheet.
A further problem with the prior art fusers is the tendency of the image receptor to curl around one of the rollers as the image receptor passes from between the rollers. This requires that a "paper finger" be set up adjacent the roller to pull the image receptor off the roller and guide the image receptor out of the fuser. The "page fingers" must contact the rollers in order to pluck the image receptor from the roller to which it is attracted. This contact with the roller causes undue wear and, eventually, uneven roller surfaces
Another problem with the prior art fusers is the difficulty involved in repairing or servicing the fuser. Prior art fusers are fixed within the print engine and are not easily accessible. This problem is particularly annoying when the image receptor, usually paper, jams in the fuser. These jams and other repairs to the fuser are usually beyond the servicing ability of the end consumer. Fuser servicing is done by a "key operator" who must come to the site of the print engine to effect repairs or requires the print engine to be brought to a service center. This results in down time when the print engine cannot be used for the purpose for which it was purchased.
Still another problem with the prior art fusers is the difficulty of the replacement of the oil application means. Prior art oil wicks, tanks and rollers have to be replaced often. These oil wicks and rollers are coated in oil. When removed, the prior art wicks, rollers and oil tanks tend to spill oil inside the machine. Accordingly, replacement of the oil wicks, tanks and rollers is often difficult and sloppy.
Another problem with prior art fusers is an uneven fixing roller surface caused by wear on the outer sleeve of the fixing roller. It is difficult to apply an even oil film to a fixing roller having an irregular surface. Also, a fixing roller with an irregular surface tends to apply an uneven layer of oil to the image receptor as the image receptor passes between the fixing roller and the pressure roller. Further, a fixing roller having an irregular surface does not apply uniform pressure to the image receptor as the image receptor passes between the fixing roller and the pressure roller. Accordingly, a fixing roller having an irregular surface does not uniformly fuse the toner image to the image receptor sheet and often causes non-fused streaks in the final toner image.
Leveling blades in prior art fusers tend to catch and bind against the fixing roller as the fixing roller passes by the leveling blade. The binding leveling blade can inflict considerable damage upon the outer surface of the fixing roller leading to an uneven surface of the fixing roller. Leveling blades are most likely to bind against the fixing roller when there is an inadequate amount of oil on the outer surface of the fixing roller.
Toner which accumulates on the outer surface of the fixing roller can also damage the outer surface of the fixing roller. As the fixing roller fuses toner-laden image receptor sheets, particles of toner are often "offset" from the image receptor sheet onto the outer surface of the fixing roller. This offset toner tends to accumulate on the outer surface of the fixing roller adjacent the leveling blades, oil wicks, or oil rollers. The accumulated toner causes the most damage when the fixing roller has cooled while the fuser is not operating, thereby hardening the accumulated toner on the outer sleeve of the fixing roller. When the fuser first resumes operation, the hardened toner can be forced under an oil wick, an oiling roller, a leveling blade, or between the fixing roller and the compression roller and damage the outer surface of the fixing roller.
The problems associated with offset toner are more severe in a color print engine. The upper layers of toner on the image receptor sheet are not held to the image receptor sheet with as much electrostatic force as the lower layers of toner. As a result, the upper layers of toner are more likely to offset from the image receptor sheet and adhere to the outer surface of the fixing roller.
In general, oil wicks tend to wear unevenly and also tend to accumulate a film of offset toner along the line of contact between the oil wick and the fixing roller. The offset toner which accumulates on the oil wick tends to block the flow of oil from the oil wick to the outer surface of the fixing roller, thereby causing the oil wick to apply an uneven oil film to the fixing roller.
Therefore, there is a need for a fuser for use in an electrophotographic print engine wherein multi-color toner images are uniformly fused. There is also a need for a fuser wherein the image receptor does not curl around one of the rollers as the image receptor passes from between the rollers. There is also a need for a fuser that is easily accessible for repair and wick replacement. There is also a need for a fuser wherein the wear on the fixing roller is reduced.